CF4, C2F6, NF3 and SF6 are commonly used in the semiconductor and flat panel display manufacturing industry, for example, in dielectric layer etching and chamber cleaning. Following the manufacturing or cleaning process there is typically a residual PFC content in the effluent gas pumped from the process tool. PFCs are difficult to remove from the effluent gas, and their release into the environment is undesirable because they are known to have relatively high greenhouse activity.
The object of abatement is to convert the PFCs into one or more compounds that can be more conveniently disposed of, for example, by conventional scrubbing.
A known PFC abatement technique is to use combustion to remove the PFCs from the effluent gas. An example of this technique is described in EP-A-O 694 735, wherein a fuel gas is mixed with a nitrogen stream containing a PFC, and that gas stream mixture is conveyed into a combustion zone that is laterally surrounded by the exit surface of a foraminous gas burner. A fuel gas and air are simultaneously supplied to the foraminous burner to effect flameless combustion at the exit surface, with the amount of air passing through the foraminous burner being sufficient to consume not only the fuel supplied to the burner but also all the combustibles in the mixture injected into the combustion zone. Premixing the PFC with a fuel prior to the entry of the stream into the combustion zone was found to improve the efficiency of the PFC abatement. Good results were obtained with C2F6, SF6 and NF3, but the technique was not applicable to the abatement of CF4 due to the maximum temperatures attainable within the combustion zone.
A modification of the above technique is described in EP-A-O 802 370, in which the gas stream mixture is conveyed into the combustion zone through a nozzle that is concentric with a lance that introduces oxygen into the mixture before it enters the combustion zone. Using this technique, good results were achieved for all PFC gases, including CF4.
Recently, chamber cleaning has moved away from PFCs such as C2F6, CF4 to plasma dissociated NF3. Plasma dissociation of NF3 produces high concentrations of extremely reactive atomic and molecular fluorine, which reacts with the debris in the process chamber to produce volatile fluorides, such as SiF4. As only a small percentage of the fluorine reacts with the debris in the process chamber, the effluent stream from the process chamber contains large quantities of toxic and reactive fluorine, as well as significant quantities of un-reacted NF3. In view of this, the aforementioned techniques are not suitable for the abatement of such an effluent stream, as premixing a fuel into an effluent stream containing fluorine would result in a spontaneous exothermic reaction.
Furthermore, for large area semiconductor cleaning, the effluent fluid stream from the process chamber typically contains not only very high concentrations of PFCs such as SF6 and/or CF4 but also significant amounts of O2 and Cl2 as oxidants for achieving high chamber cleaning rates and high fractional utilisation of the cleaning gases. It is not possible to abate these mixtures to satisfactory levels using the techniques described above, as pre-mixing a hydrocarbon fuel with an effluent stream containing high concentrations of O2 and Cl2 produces a significant risk of flashback and combustion within the exhaust piping. In addition, the free radical quenching nature of Cl2 further suppresses the reaction of the PFCs, leading to low abatement efficiencies.
More recently, dielectric layers are moving towards materials with lower dielectric constants (“low-k materials”). The precursors for these materials include 3MS (trimethylsilane (CH3)3SiH), 4MS (tetramethylsilane (CH3)4Si), DMDMOS (dimethyldimethoxysilane (CH3O)2Si(CH3)2), TMCTS (tetramethylcyclotetrasiloxane (CH3(H)SiO)4) and OMCTS (octamethylcyclosiloxane ((CH3)2SiO), which are organo-silanes with a significant carbon content. These chemicals can often be difficult to abate as they tend to be flammable in comparison to silane, which is pyrophoric. They are also much less reactive, and so precise combustion conditions are often needed to avoid the formation of products of incomplete combustion, which products can vary from solids and gels to foams.
It is an aim of at least the preferred embodiment of the present invention to seek to provide abatement apparatus that has a configuration which enables the apparatus to be used for a wide range of process gas mixtures.